1. In a small signal circuit, the resistance of a very short copper wire must not be important, right?
Conductive wires of PCB are comparatively wide, and gain error can be reduced. It is better to use comparatively wide conductive wires in analog circuits, but many PCB designers prefer to use conductive wires of minimum width to facilitate the arrangement of signal wires. In short, for all sections where problems may appear, it is important to calculate conductive wires resistance and analyze it.
2. Is there is a problem of capacitance formed by wide conductive wires and the metal layer on the back of a printed circuit board?
A small problem. Capacitor formed by PCB conductive wires should be calculated. If a problem does exist, a small area of the ground plane can be removed to reduce the capacitance to ground.
3. What is ground plane?
If the entire side copper foil of a printed circuit board (or the entire layer copper foil of a multi-layer printed circuit board) is used for grounding, then this is a ground plane. The layout of ground wires should ensure that the ground wires have minimal resistance and inductance possible. If a system uses one ground plane, it is almost not affected by grounding noises. And the ground plane has the function of shielding and heat dissipation.
4. If system which uses one ground plane is almost not affected by grounding noises, what is the ground noise problem?
Although one ground plane has resistance and inductance, external current is strong enough to influence precise signal. Through reasonable layout of PCB, big current cannot flow into the section where accurate signal will be affected. Sometimes a break or a crack in the ground plane can cause a large ground current to change the direction of the flow from the sensitive area, but changing the ground plane violently can also cause the signal to flow into the sensitive area, so this technique must be used with caution.
5. How do you know the voltage drop that occurs on a ground plane?
Generally, voltage drop can be measured. Voltages in the DC to low frequency (50kHz) range can be used with instrumentation amplifiers. If instrumentation amplifiers are separated with their power ground, the oscilloscope must be connected to the power ground of its power circuit. The resistance between any two points on the ground plane of LED lighting can be measured with a probe. The combination of amplifier gain and oscilloscope sensitivity results in a measurement sensitivity of 5μV/div. Amplifier noise can increase the width of oscilloscope
wave curve, and the width is about 3μV. However, it is still possible to achieve a measurement resolution of about 1 μV, which is sufficient to identify most ground noise, with a confidence level of up to 80%.
6. How is high-frequency ground noise measured?
Measuring high-frequency ground noise with a suitable broadband instrumentation amplifier is difficult, so it is appropriate to use high-frequency and VHF passive probes that consist of ferrite magnetic ring (outer diameter 6~8mm). There are two coils on the magnetic ring, and each coil has 6~10 turns wires. One coil is connected to the spectrum analyzer input and the other coil is connected to the probe to form a high-frequency isolation transformer. The method to test high-frequency isolation transformers is similar to the method to test low-frequency isolation transformers. However, spectrum analyzers use amplitude-frequency characteristic curves to represent noise, which is different from time-domain characteristics, where noise sources can be easily distinguished according to their frequency characteristics. In addition, the sensitivity of a spectrum analyzer is at least 60dB higher than that of a broadband oscilloscope.
